Concrete is capable of withstanding significant compressive loads, however, it is not as capable of withstanding significant tensile loads. Thus, it is often necessary to reinforce concrete structures with steel bars, cables, or the like to enhance the structure's ability to withstand tensile forces.
The basic principles of providing such reinforcement to concrete structures are known in the prior art. In a post-tensioned reinforcement system, several steel cables (called “tendons”) are placed within the concrete framing structure where the concrete will later be poured around them. At this point, each tendon is held loosely in place, and the ends of each tendon pass through an anchor on each side of the concrete slab that composes a portion of the total concrete structure. Once the concrete is poured and has cured for a sufficient amount of time, but not yet to the point of being fully cured, the tendons may be tensioned by a hydraulic tensioner. The hydraulic jack tensioners that may be used in these circumstances are driven by high pressure hydraulic fluid in one or more cylinders in the tensioner that places the tendon under a high tensile load, for example 30-40,000 pounds force.
A concrete anchor is typically formed as a singular body by casting and includes a body portion that has a flange portion, and two generally cylindrical shaped portions, one extending from the front surface of the flange (nose portion) and one extending from the rear surface of the flange (button portion). To help support the force that will be applied to the tendon after tensioning, the anchor also includes several ribs located on the front surface of the flange. The rear surface of the flange is used to contact the concrete and provide a load bearing surface during the tensioning of the tendon by the hydraulic jack tensioner. The flange portion of the anchor is typically of a constant thickness and includes two or more mounting holes so the anchor can be fastened to the concrete structure, which is often completed with nails or similar fasteners.
The anchors in the prior art also include a bore that extends through the nose portion, the flange portion, and the button portion. The diameter of the bore decreases along the axis of the bore in the direction from the front surface to the rear surface of the flange, with the largest diameter being near the front side of the nose portion. Because of the decreasing diameter, the bore is capable of receiving a wedge that surrounds the tendon.
Before the concrete is poured around the tendons, each tendon must pass through an anchor that will be located on each side of where the concrete slab will eventually be located. The tendon enters the anchor by entering the bore in the button portion on the rear surface of the flange and exiting the bore in the nose portion on the front surface of the flange. After the tendon exits the anchor, the wedge may be placed around the tendon in the frusto-conical bore of the anchor.
After the concrete is poured and allowed to partially cure for a sufficient amount of time, the tendon may be tensioned by a hydraulic jack tensioner. When the tendon is tensioned by the hydraulic jack tensioner, the tendon and wedge are forced tightly into the bore. The wedge is shaped such that it has teeth that help lock the tendon in place during tensioning. The tensioning force on the tendon passes to the wedge and to the nose, button, and flange portions of the anchor, and ultimately to the concrete slab. The ribs help distribute that force throughout the body of the anchor and onto the rear surface of the flange portion of the anchor, thus providing the tensile strength to the concrete structure. After tensioning, the anchor may be kept external to the concrete structure, or alternatively, a concrete section may be poured outside of the anchor to encapsulate the anchor.
Many anchors are used in post-tensioned concrete construction and so it is important that they be efficient in performing their function. Excess material in an anchor leads to increased weight that is unnecessary for the proper functioning of the anchor and results in a waste of raw materials used to compose the concrete anchor, as well as increased shipping costs of the anchor.